fix(#94): replace text-white with text-primary-fg on all primary buttons

In dark mode --c-primary switches from navy (#012851) to mint (#a1dcd8).
Buttons using bg-primary+text-white showed white text on mint at 1.4:1
contrast — invisible. bg-brand-navy buttons were also invisible (navy on
near-black canvas, 1.3:1).

Replaced in 28 components app-wide:
- bg-primary ... text-white → text-primary-fg
- hover:bg-primary hover:text-white → hover:text-primary-fg
- bg-brand-navy ... text-white + hover:bg-brand-navy/90 →
  bg-primary ... text-primary-fg + hover:bg-primary/90

Light mode is unchanged: primary-fg = white in light mode.
Dark mode: primary-fg = navy (#012851) on mint bg = readable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/DocumentTopBar.svelte

@@ -102,8 +102,8 @@ const compactMeta = $derived.by(() => {
 				onclick={() => (annotateMode = !annotateMode)}
 				aria-label={annotateMode ? m.doc_panel_annotate_stop() : m.doc_panel_annotate()}
 				class="flex items-center gap-1.5 rounded px-3 py-1.5 font-sans text-xs font-medium transition {annotateMode
-					? 'bg-primary text-white'
-					: 'border border-primary text-ink hover:bg-primary hover:text-white'}"
+					? 'bg-primary text-primary-fg'
+					: 'border border-primary text-ink hover:bg-primary hover:text-primary-fg'}"
 			>
 				<img
 					src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Note/Note-Add-MD.svg"
@@ -118,7 +118,7 @@ const compactMeta = $derived.by(() => {
 		{#if canWrite}
 			<a
 				href="/documents/{doc.id}/edit"
-				class="flex items-center gap-2 rounded border border-primary bg-transparent px-3 py-1.5 text-xs font-medium text-ink transition hover:bg-primary hover:text-white"
+				class="flex items-center gap-2 rounded border border-primary bg-transparent px-3 py-1.5 text-xs font-medium text-ink transition hover:bg-primary hover:text-primary-fg"
 			>
 				<img
 					src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Edit-Content-MD.svg"